ed25519-zebra 2.2.0

Zcash-flavored Ed25519 for use in Zebra.
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Zcash-flavored Ed25519 for use in [Zebra][zebra].

Zcash uses Ed25519 for [JoinSplit signatures][zcash_protocol_jssig] with
particular validation rules around edge cases in Ed25519 signatures.  Ed25519,
as specified in [RFC8032], does not specify behaviour around these edge cases
and so does not require conformant implementations to agree on whether a
signature is valid.  For most applications, these edge cases are irrelevant,
but in Zcash, nodes must be able to reach consensus on which signatures would
be valid, so these validation behaviors are *consensus-critical*.

Because the Ed25519 validation rules are consensus-critical for Zcash, Zebra
requires an Ed25519 library that implements the Zcash-flavored validation rules
specifically, and since it is unreasonable to expect an upstream dependency to
maintain Zcash-specific behavior, this crate provides an Ed25519 implementation
matching the Zcash consensus rules exactly.

However, this library may be of independent interest, as it implements
ZIP215, a set of precisely specified validation rules for Ed25519 that make
individual verification consistent with batch verification and are
backwards-compatible with all existing Ed25519 signatures. Any non-Zcash users 
should use the ZIP215 rules:
```toml
ed25519-zebra = "2"
```

## ZIP 215 and changes to Zcash-flavored Ed25519

[Zcash Improvement Proposal 215][ZIP215] changes validation criteria for
Ed25519 signatures in Zcash after its activation (currently scheduled for the
Canopy network upgrade at block height 1046400). These changes remove the
dependence on validation rules inherited from a specific point release of
`libsodium` and make individual verification consistent with batch
verification. More details and motivation are available in the text of [ZIP215].

The `1.x` series of this crate implements the legacy, pre-ZIP-215 validation
criteria; the `2.x` series of this crate implements the post-ZIP-215
validation criteria.  Users (like Zebra or Zcashd) who need to handle the
upgrade can use both versions simultaneously using cargo renaming, e.g.,
```toml
ed25519-zebra-legacy = { package = "ed25519-zebra", version = "1" }
ed25519-zebra-zip215 = { package = "ed25519-zebra", version = "2" }
```

## Example

```
use std::convert::TryFrom;
use rand::thread_rng;
use ed25519_zebra::*;

let msg = b"Zcash";

// Signer's context
let (vk_bytes, sig_bytes) = {
    // Generate a signing key and sign the message
    let sk = SigningKey::new(thread_rng());
    let sig = sk.sign(msg);

    // Types can be converted to raw byte arrays with From/Into
    let sig_bytes: [u8; 64] = sig.into();
    let vk_bytes: [u8; 32] = VerificationKey::from(&sk).into();

    (vk_bytes, sig_bytes)
};

// Verify the signature
assert!(
    VerificationKey::try_from(vk_bytes)
        .and_then(|vk| vk.verify(&sig_bytes.into(), msg))
        .is_ok()
);
```

[zcash_protocol_jssig]: https://zips.z.cash/protocol/protocol.pdf#concretejssig
[RFC8032]: https://tools.ietf.org/html/rfc8032
[zebra]: https://github.com/ZcashFoundation/zebra
[ZIP215]: https://github.com/zcash/zips/blob/master/zip-0215.rst